1. Field of the Invention
The present invention relates to a method whereby environmental pollutants can be eliminated from the polluted environment (polluted sites) such as, for example, soil or groundwater which have been contaminated with environmental pollutants.
2. Description of the Related Art
Recently, leaks into the environment of chlorinated organic solvents such as trichloroethylene and dichloroethylene that have been used as detergents in many factories are arising a great concern among public. In the United States, environmental pollution such as the contamination of soil and groundwater with chlorinated organic solvents, initiated by the contamination with petroleum and gasoline, have progressed to a gravely risky degree. To meet such situations, general tendencies proceed towards imposing sterner regulations onto the use of such chlorinated organic solvents as mentioned above. However, a problem still remains how to treat the environment that has been contaminated with those chlorinated organic solvents, that is, how to treat the soil and groundwater contaminated with such pollutants.
To cope with such problems, methods for treating soil and groundwater contaminated with chlorinated organic solvents have been investigated. The treating methods hitherto proposed consist of physical processing such as air extraction under vacuum or combustion, but there is still a strong demand for methods whereby the environment, without suffering undue stresses, can be cleaned up such pollutants at a low cost. To meet such demand, the technologies enabling the decomposition/elimination of environmental pollutants by the use of microorganisms or so-called "bio-remediation" of the polluted environment have been proposed and are attracting people's attention.
Bio-remediation of the polluted environment can be achieved by roughly three different procedures: 1) a bio-reactor type of treatment which consists of fixing bacteria that decompose pollutants and of applying them as a solid (a closed-system treatment), 2) a treatment which consists of activating the bacteria endogenous to the polluted soil or groundwater by providing them with various nutrients (endogenous bacteria activation) and 3) a treatment which consists of delivering directly to the polluted sites bacteria (they may be delivered in the form of solids) capable of decomposing pollutants (an open-system treatment). The characteristics of the three methods are summarized in the table below.
TABLE 1 ______________________________________ Endogenous Closed-system bacterial Open-system treatment activation treatment ______________________________________ Installation Big Intermediate Small cost Running cost Big Intermediate Small Treatment time Small Intermediate Big Treatable amount Intermediate Big Intermediate Reaction controllability Easy Difficult Almost impossible Bio-safety Big Small Small Concentration of Big Intermediate Small pollutants to be treated ______________________________________
The conventional methods such as described above which concern with the decomposition/elimination of environmental pollutants by the use of microorganisms have their characteristic disadvantages as well as advantages as shown in Table 1. Take the close-system treatment as an example, it is excellent in treating capacity and in easy controllability and enjoys high safety while it is disadvantageous in that its installation and operation require a high cost. The endogenous bacteria activation, on the other hand, is excellent in treating capacity, but is disadvantageous in reaction controllability, cost-effective installation and operation, and bio-safety. The open-system treatment, though being meritous in enabling low-cost installation and operation, and in achieving biosafety, is still deficient in treatment time and in treatable pollutant concentrations. Furthermore, with all these methods reactions are practically uncontrollable, and inevitable addition of microorganisms to existent bacteria may become a threat to the bio-safety of the environment.
Because, as are discussed above, conventional methods for decomposing/eliminating environmental pollutants by the use of microorganisms have their own characteristic advantages and disadvantages at the same time, it is customary currently to choose one from among them and to apply it in combination with other chemical or physical methods, according to the nature of the pollutants of the environment to be treated.
The bio-remediation has its characteristic merits: it requires only a low cost for operation, it doesn't require much man-power for operation/maintenance, and it doesn't impose much stresses on the environment. The open-system treatment appears to be most amply provided with such merits characteristic to bio-remediation, but there are still strong demands that the above-described defects characteristic to the open-system treatment should be corrected further.